The invention relates to an infrared detector device for detecting properties of a moving web of paper, more specifically for use in controlling production in a paper making process, the detector device being provided, on one side thereof, with a polychromator that receives light coming from the web of paper and, on the other side, with an infrared detector having a matrix of infrared sensitive photoconductors, and an evaluation electronic unit with a separate memory assigned to each of the photoconductors of the matrix being connected downstream thereof. The invention also relates to a method for operating such an infrared detector device.
A device of this type is described in PCT/DE 99/02530. Such matrix detectors are mainly utilized for image digitization; for example, they are known to have been used for night viewing apparatuses. They consist of a matrix of individual pixels, each pixel being a photoconductor. By photoconductor is meant a photosensitive electrical element that generates an electric current or voltage change when illuminated. More specifically, possible photoconductors are infrared sensitive semi-conductors, e.g., PbS, CCD, and CMOS arrays.
The problem with infrared detectors of the type mentioned above is that the individual pixels or photoconductors have differing properties. In particular they tend to differ in offset and sensitivity.
With the infrared detector device of the type mentioned above, it is important for the result of the measurement that the behavior of each and every one of the pixels of the detector matrix is the same as that of all the others. The web of paper is sensed by regions for detecting defects or deviations thereof. Sensing is carried out in what are termed measuring dots. Usually, a great number of such measuring dots is arranged over the width of the conveyed web of paper. At least one pixel, preferably a complete line or row of pixels, is assigned to a respective one of the measuring dots. Their properties are crucial to the result of the test for determining whether a web of paper meets the required standards or not.
In view of the above, it is the object of the invention to improve an infrared detector device of the type mentioned, and to provide a method for operating such a device, aimed at equalizing the behavior of the pixels in the result of the measurement and at conferring controllable properties onto the pixels. More specifically, the invention aims at excluding pixels below standard, which cannot be used in practice for any reason, more specifically because they are too insensitive, so that these pixels do not influence the result of the measurement.
The solution to this object, with regard to the device, is to provide it with an additional source of light that can be controllably switched on and off between a first light intensity and a second light intensity which is greater than the first one. The additional source uniformly illuminates the infrared detector and is arranged so that it does not hinder (at least not when in its switched-off condition) the beam path of the light passing through the polychromator and illuminating the infrared detector.
In accordance with the invention, a method for operating an infrared detector device as specified in the previous paragraph is provided, in which the additional source of light is operated at the first light intensity and the thereby obtained electrical signals of every photoconductor are acquired, in which the additional source of light is operated at the second light intensity and the thereby obtained electrical signals of every photoconductor are again acquired, and in which, by comparison with a reference pixel, first and second correction values for each of the photoconductors are obtained based on the electrical signals. The two correction values for every photoconductor are saved and taken into consideration in later measurement.
Thus, in accordance with the invention, the detector matrix is homogeneously illuminated by means of an additional light source. This occurs out of a measuring phase, in what is termed a test phase. During the test phase, no other light, no entering light in particular, is allowed to be incident upon the detector matrix. If necessary, such a light is shaded. If no suited means, such as a chopper for example, is provided, it is advantageous to provide for an appropriate device such as a shutter or a closure for example.
In accordance with the invention, each pixel has been assigned a memory of its own. Within a greater memory, a certain address is assigned to only that specific pixel. The correction values obtained in the test phase are saved in the memory and are used to correct measured values of the signals of the individual pixels obtained later on in the measuring phase, the corrected measured values then being outputted.
With regard to the method, the test phase is carried out in the light and in the dark. In the light, two different light intensities are used, i.e., a first light intensity and a second light intensity, which is greater than the first one. Preferably, the two light intensities are within the range of the light intensity generated by the entering light onto the respective pixels. Accordingly, the first light intensity preferably is within the lower intensity range of the entering light and the second light intensity preferably within the upper intensity range of the entering light to be measured. The additional source of light is operated accordingly.
The correction of every single pixel is made according to a linear equation, i.e., according to a line equation with a gradient as a first correction value and with an offset or Y-axis intercept as a second correction value. Correction is made for the electrical signals of every single pixel obtained both in the dark and in the light.
Preferably, an infrared light emitting diode (IR-LED) is used as the additional source of light. It is preferably associated with a temperature control device that keeps the temperature of the diode constant. The temperature is preferably below room temperature, so that the LED is being cooled. In a preferred embodiment, this is achieved by means of a Peltier element which is assigned a temperature sensor and a control circuit.
In a preferred embodiment, the additional source of light is located inside a polychromator. It is arranged in such a manner that it directly illuminates the detector matrix through the exit window. It is located at a distance of some centimeters from the detector matrix and in the beam path behind the element in the polychromator that splits the light, i.e., a grating or a prism.